Mathematical Physics: Classical Mechanics
《Mathematical Physics: Classical Mechanics》 · 第 110 页 · Chapter 11，The Fundamental Group 开头（补空间基本群义）
The group of a link \(L\) in \(S^3\) is defined to be \(\Pi_1(S^3-L)\), the fundamental group of the complement of \(L\); this is the same as \(\Pi_1(X)\), where \(X\) is the exterior of \(L\).